FIELD OF THE INVENTION
The invention relates to a method and a device for releasing a sheet from a blanket cylinder of a printing machine, wherein the sheet is guided by the impression cylinder and held by grippers thereof.
In offset printing, ink is applied by a plate cylinder to a blanket cylinder and is, in turn, printed by the latter onto a sheet located on an impression cylinder. The application of ink to the sheet occurs at a line of contact between the blanket cylinder and the impression cylinder. This is called the printing nip. Adhesion forces are produced between the sheet and the blanket cylinder by the ink, causing the sheet to adhere to the blanket cylinder. After a given adhesion length or distance on the blanket cylinder, a tear-off line is reached at which the sheet is pulled away from the blanket cylinder. The angle at the blanket cylinder between the printing nip and the tear-off line is called the follower or co-rotation angle and is usually about 5.degree.. The greater the follower angle, the greater is also the pull-off angle, which is the angle that the paper sheet forms at the tear-off line to the tangent to the blanket cylinder. This run-on of the sheet has been considered to be disadvantageous in the state of the prior art, and measures have been devised to reduce this sheet run-on, due to which the pull-off angle has been reduced.
Thus, the published German Patent Document DE 43 08 595 A1 addresses the objective of developing a method and a device for reducing the sheet run-on, which permit the sheet to be tautened to as far as the printing nip. For this purpose, this German patent document proposes that the circumferential speed be set in such a manner that the tensile forces on the sheet are as high as possible after the sheet leading edge has passed the printing nip.
The published German Patent Document DE 32 20 364 A1 also follows such a path. The objective thereof is to make a sheet tensioning device available which displaces the tear-off line of the sheet from the cylinder as closely as possible to the printing zone. For this purpose, the gripper system on the impression cylinder has been equipped with spring-controlled and cam-controlled levers, which permit the gripper system to be locked in a specific position and to be unlocked at a later time, in order to apply a tensile stress to the sheet in the running or travel direction thereof. Thus, the tensile stress which acts upon the sheet in any case was additionally increased after the sheet leading edge passed the printing nip, in that a high spring force was released, which additionally tautens the sheet. What was intended to be achieved in this manner was that the tearing line of the sheet be displaced as closely as possible to the printing zone, in order to avoid, as best as possible, the adhesion of the sheet to the blanket cylinder.
The disadvantage of this operating method and of the corresponding devices, respectively, is that measures which are directed to displacing the tear-off line as close as possible to the printing nip lead to the result that the pull-off angle is reduced and, accordingly, the tensile force which acts upon the sheet must be increased in order to attain the force component which is necessary for the release of the sheet. Increasing the forces acting upon the sheet leads to a great mechanical effort having to be expended in order to equip the grippers with adequately high holding forces. In addition, this leads to the production of markings by the grippers on the paper sheet, or the paper sheet being deformed by the increased tensile forces which, in turn, has a detrimental effect upon the printing quality due to the occurrence of ghosting. Also, in spite of the high gripper forces, it is often not possible to prevent the paper sheet from being drawn out from the grippers a given distance which, in turn, has the aforementioned negative influence upon the printing result.